1. Field of the Invention
The present invention relates to optical circuits realized by means of planar technology and, more particularly, to an integrated planar optical structure.
2. Description of the Related Art
A typical field of application of planar lightwave circuits (PLC) is constituted by optical communication systems, inclusive of the modern transmission systems that exploit wavelength division multiplexing (WDM or DWDM). The greater part of the optical devices necessary for the transport and processing of the light signals, such as amplifiers, modulators, attenuators and so forth, are realized as planar integrated lightwave circuits.
In a planar lightwave circuit the propagation path of the light is defined by a core obtained from a layer, known as the core layer, comprised between two cladding layers. The materials are chosen in such a manner as to obtain a refraction index of the core layer greater than that of the cladding layers and to render possible the total reflection of the light to be transmitted.
Typically, planar lightwave circuits are fabricated by utilizing the techniques commonly employed for the fabrication of integrated electronic circuits, such as the deposition of layers of different or differently doped materials and photolithographic processing. In a known process a first layer of silicon dioxide with a predetermined type and/or concentration of dopant is deposited on a silicon substrate. In this way one obtains the lower cladding layer. The next step is the deposition of the core layer. In this case it consists once again of silicon dioxide, but with a dopant type and/or concentration different from those of the first layer. This is followed by the deposition of photoresist and the formation of a photolithographic mask in accordance with the layout of the proposed circuit. The next step is an attack to etch the core layer in the areas not protected by the mask. After the photoresist mask has been removed, there remain the cores that will constitute the lightwave circuit. The final step consists of the deposition of a second and last cladding layer (upper cladding layer). This last layer consists once again of silicon dioxide and has the same dopant as the first layer. The refraction indices are established in the design phase by appropriately choosing the dopant type and concentration of the core layer and the cladding layers. In this way one also defines the effective refraction index of the circuit, which, depends on the geometry of the designed waveguide and the difference between the refraction index of the core and that of the cladding. With its core and the part of the cladding adjacent to the core, a waveguide defines an optical path. A light signal applied to the entry of the waveguide becomes propagated along the light path as guided energy, but, due to the effect of dispersion, is also partly radiated into the cladding.
In a communication system it is important that the transmitted signal should remain as unaltered as possible, i.e., as free from attenuation as possible. Obviously, it is impossible for the signal not to become disturbed by noise or not to be subject to power reductions, so that whenever the optical path is very long, several kilometers for example, the output signal may be altered and greatly attenuated. With a view to obviating this drawback, there are provided not only such optical devices as amplifiers or repeaters, but also monitoring systems that make it possible to have information about the state of the light signal at the beginning and the end of the optical path and also along its course.
A typical case is the monitoring of the light energy emitted by a laser source coupled directly to an optical device realized with planar waveguides and intended to be transmitted by means of, for example, a single-mode optical fiber. The laser source is mounted in the vicinity of the entry of a waveguide. The light energy enters the waveguide and, in a typical case, passes through a section of the waveguide to reach a WDM (Wavelength Division Multiplexer) structure capable of separating a signal associated with the light energy into components of different wavelengths. The components then continue in optical fiber through the subsequent network architecture.
A technique for monitoring the useful power of the light beam emitted by the laser source envisages the introduction of a collector device between the source and the beginning of the optical fiber. The device directly collects a fraction of the useful power and sends it to a photodiode connected to appropriate processing equipment. But this technique introduces a loss, because it calls for the removal of a fraction of the useful power that should enter the optical fiber. This loss has to be added to the others already present, such as, for example, the loss due to laser alignment errors. It is extremely difficult, if not altogether impossible, to align the laser in such a way that 100% of the light beam will enter the waveguide. In actual practice the light, already at the beginning, is partly directed into the waveguide and partly radiated into the cladding of the PLC and therefore lost. It is estimated that laser alignment problems cause the dispersion of almost 50% of the power of the emitted signal. Another difficulty is due to the spectral behavior of the WDM structure, possibly integrated in the PLC, which nominally should be independent of the emission wavelength of the source, though this is not so in actual fact. While it functions, the laser source is subject to temperature variations that imply mechanical dilations of the sources and modify the wavelength of the emitted light. The WDM structure responds differently every time that the wavelength varies following a variation of the temperature.
In the case here described the energy radiated into the cladding of a PLC constitutes a loss that has to be taken into account in the design of a communication network. In other applications the energy radiated into the cladding is a desired dispersion in order to comply with the design specifications of an optical device. For example, the optical parameters of a waveguide may be chosen in such a manner that, at a given frequency, the waveguide will permit only the propagation of the fundamental mode, while all the other modes, known as evanescent modes, are dispersed into the cladding. Even in the attenuation devices a part of the signal that arrives as input is propagated as guided energy in the core and a part is radiated into the cladding and dispersed.
A device of this latter type, known as a variable optical attenuator (VOA), for use in wide-band applications with a higher attenuation is realized by connecting two Mach-Zehnder (MZ) interferometers in cascade by means of a waveguide. An MZ interferometer consists of an input Y-coupler, i.e., an optical coupler that divides the incoming light energy into two equal parts, two waveguide branches that guide the two parts of the light energy into different light paths by introducing a predetermined phase difference between the two signals and an output Y-coupler. The phase difference may be obtained by realizing two branches of different lengths or by forming two electrodes on one of the two branches and applying to them a voltage such as to modify the refraction index of the light path between the two electrodes due to thermo-optical and/or electromagnetic effects. The two signals will therefore be out of phase and will be summed in constructive or destructive interference. The output signal of the first stage is partly conveyed into the waveguide between the first and the second stage as guided energy and partly dispersed into the cladding. But the energy thus radiated into the cladding is not wholly dispersed at the output of the first stage, because a part succeeds in reaching the beginning of the second stage, so that when it eventually reaches the input Y-coupler of the second stage, it becomes once more coupled with the fundamental mode in the waveguide and is thus added to the guided energy within the light path. The attenuation of the input signal introduced by the twin stage is not therefore as expected, because the light energy lost by the first stage is partly recovered by the second. The design of the attenuator therefore becomes somewhat difficult and has to take account of this phenomenon.
A first known solution of this problem is to prolong the part of the waveguide between the two stages and therefore the distance between them. This solution has the drawback of increasing the encumbrance of the optical device.
A second known solution is to create discontinuities in the zone between the two stages in the form of trenches that partly reflect the radiated energy. This solution calls for supplementary processing to create the trenches and particular attention has to be paid to valuing the exact position of the trenches with respect to the propagation waveguide: if the hollows are too close, the propagation of the guided energy is disturbed; if they are spaced too far apart, some of the radiated energy passes the trenches and reaches the second MZ stage all the same.
Another known solution is to realize an absorption structure to be interposed between the two stages, creating trenches similar to the ones of the solution that has just been described and filling the trenches with metal or depositing a metallic structure directly on the PLC in a subsequent processing phase. In this case the drawbacks are constituted by the supplementary processing that is required and the disturbance that the metallic structure may cause to the propagation of the guided energy. Indeed, any metallic structure in the vicinity of a waveguide can excite propagation modes and become a waveguide in its turn, thereby increasing the signal interference level.
In a planar lightwave circuit (PLC) in which there are realized one or more light paths to guide the light energy of a signal applied to the circuit there is often present some energy that is radiated into the cladding and will not therefore be guided.